Most of the soil in dry regions or coastal areas on earth contains salts and therefore is not deemed favorable for vegetation. There are problems that crops cannot grow in such lands, or cannot reap sufficient harvests even if the crops can grow there. However, development of crops which can grow in such lands has been eagerly expected to deal with expansion of dry regions attributable to global warming and to deal with population increases in developing countries. Accordingly, there is an urgent need to develop salt-tolerant crops by means of breeding or genetic engineering.
Meanwhile, most of plants growing on earth are exposed to various environmental stresses such as high temperature, low temperature, dry weather, and high salinity. The plants continue to grow by exerting resistance to those stresses in some way. To be more precise, it is known that various stress response genes operate when a plant is exposed to the foregoing environmental stresses, and the plant shows resistance to the environmental stresses by performing physiological responses at the cellular level. The genes considered to cause the plant to exert such a function have been already isolated by a subtraction hybridization method and by a differential screening method.
However, it is considered that there are enormous numbers of genes responsive to environmental stresses as the plants show various inductions of gene expressions and inhibition patterns depending on differences in environmental stress factors and in plant species. Accordingly, the present situation is still far to isolation of all the relevant genes, and isolation of these genes is now in the process of energetic efforts.
Meanwhile, there are two methods to produce or breed a salt stress tolerant plant, namely, a method of producing a salt stress tolerant transgenic plant by artificially introducing a gene related to salt stress tolerance into a plant cell, and a method of introducing the gene taken from a salt stress tolerant plant into a plant targeted for breeding by use of crossing technologies. In order to realize these, it is necessary to isolate the gene(s) related to salt stress tolerance and clarify functions thereof.
Some genes which are subjected to induction of expression under salt stress have been known to exist to date (Yao, A., Molecular biology of salt tolerance in the context of whole-plant physiology, J. Exp. Bot., 49, 915-929 (1998), Nelson, D. E., Shen, B., and Bohnert, H. J. Salinity tolerance-mechanisms, models and the metabolic engineering of complex traits, Genetic Engineering, 20, 153-176 (1998)). However, in order to develop a plant having higher tolerance, it is considered to be important to isolate more genes related to salt stress tolerance to progress in functional analyses thereof.